Diabetic radiculoneuropathy: clinical patterns of sensory loss and distal paresthesias

Effect of Heating on Nerve Conduction in Carpal Tunnel Syndrome

The effect of temperature on normal nerves is well known, i.e., an increase in conduction velocity and a decrease in amplitude with an elevation in temperature. There are few reports examining the effect of temperature on abnormal nerves, e.g., in demyelination. To study the effect of increased temperature on demyelinating nerves in entrapment syndromes, the authors investigated 55 median and 48 ulnar nerves of 48 patients with carpal tunnel syndrome, and 48 median and 48 ulnar nerves of 26 healthy subjects. All measurements were obtained at 32 degrees C and 37 degrees C. Mean reductions in median sensory amplitude occurring with heating were significantly greater in the patient group than in the control group (P = 0.000). For median sensory response amplitude, the mean decrease was 32.1% in patients with carpal tunnel syndrome and 10.7% in the control subjects. The difference between median and ulnar nerves in the latency was significantly decreased (P = 0.027) after the nerves had heated to 37 degrees C. It is concluded that the elevation in temperature leads to conduction block in demyelinated sensory nerves, and that temperature provocation may be useful in the diagnosis of nerve disorders. The effect may be different in axonal and demyelinating disorders.

Heat sensitivity of sensory fibers in carpal tunnel syndrome

Elevations in temperature may produce conduction block in demyelinated neurons. A well-described phenomenon in multiple sclerosis, it has also been reported in some patients with inflammatory demyelinating polyneuropathies. We used carpal tunnel syndrome (CTS) as a model to study the effect of heat on nerves with focal demyelination secondary to chronic compression. Compound motor and sensory responses were measured in 12 CTS patients and 12 normal subjects at 32 degrees C and with heating to 42 degrees C. Changes in relative motor response amplitude and area were similar for both normal subjects and CTS patients. In CTS patients, however, sensory response amplitude and area decreased 34.3% and 48.9%, significantly more than the 25.2% and 39.1% reductions in normal subjects (P=0.021 and P=0.018 respectively). We hypothesize that these reductions in response amplitude are secondary to the occurrence of heat-induced conduction block in demyelinated sensory neurons.

Diabetic neuropathy

The most common form of diabetic neuropathy is chronic, distal symmetrical sensorimotor, or predominantly sensory neuropathy; the latter is invariably associated with some degree of autonomic dysfunction. There are, however, other neuropathic patterns in diabetes mellitus that are uncommon but are important to recognize, since they may mimic many other non-neurologic diseases. This article discusses a variety of forms of mononeuropathies and diabetic proximal motor neuropathy, commonly known as diabetic amyotropy.

Clinical presentation and management of diabetic neuropathy and foot ulceration

A simple classification of the more common neuropathies is presented. The most frequent disorder in diabetes is a symmetrical sensory polyneuropathy in the lower limbs. Acute sensory polyneuropathies frequently follow sudden metabolic disturbance although there may be little evidence of neurological abnormalities on clinical examination. Similar symptoms occur with chronic sensory polyneuropathy but onset is gradual and this condition may persist for years with only minor symptoms. A significant proportion of patients with chronic polyneuropathies have few if any symptoms and are only diagnosed by careful clinical examination. An approach to the diagnosis and management of symptomatic sensory polyneuropathy is suggested. Sensory loss, and the possible complication of vascular dysfunction, greatly increases the risk of insensitive foot lesions in diabetic patients. Some may progress to the insensitive foot without prior evidence of neuropathy. Regular and thorough examination is therefore the only way to identify patients at risk who then require education in preventative foot care. The identification of such patients and the clinical presentation and management of foot ulcers is discussed.

Diabetic neuropathies and pain

Many of the diabetic neuropathic syndromes are characterized by painful symptoms with a sensation of burning and associated with troublesome hyperaesthesia. It is important to distinguish between the acute and chronic forms of peripheral sensory neuropathy; while the former carries an excellent prognosis for symptomatic improvement within one year, the latter may cause persistent symptoms for many years. In contrast to the acute form, in which symptoms are particularly severe but abnormal neurological signs are minimal, patchy stocking and glove sensory loss together with peripheral small muscle wasting are often present in chronic sensorimotor neuropathy. Peripheral polyneuropathies are more common in patients with poor metabolic control, although recent evidence implicates blood glucose flux as a possible contributory factor to neuropathic pain. It is possible that blood glucose flux or altered peripheral blood flow leads to increased spontaneous activity in nociceptive afferent fibres which are present in the axonal sprouts that characterize small fibre neuropathy. In the diagnosis of the neuropathies, exclusion of other aetiological factors is of paramount importance as there is no specific diagnostic test for diabetic nerve damage. If there is no symptomatic improvement after a period of stable and optimal metabolic control together with simple analgesics, then the tricyclic drugs should be regarded as first line therapy. The rapid effect of these drugs suggests a peripheral rather than central mode of action.

Diabetic neuropathy

Peripheral nerve disorders are important late complications of diabetes mellitus. Polyneuropathy, which may involve varying proportions of sensory, motor, and autonomic fibers, is considered the consequence of metabolic derangements that result from chronic hyperglycemia. Symmetrical proximal motor neuropathy ("diabetic amyotrophy") also may have a metabolic basis. Mononeuropathies in diabetes may have an ischemic or compressive cause. Advances have been made in understanding the biochemical basis for diabetic polyneuropathy. The treatment of symptomatic diabetic neuropathy should be directed toward long-term normalization of blood glucose until more specific therapies become available.